Simply refuse to allow them to 'write it off' and demand its return. However, if they cut up rough and refuse to maintain your insurance on it, you are then committing an offence subject to points and a fine, so if you are about to go on holiday you are between a rock and a hard place. You would have to take it off the road and declare a SORN to stay legal. The sticking point with the insurer is probably credit hire for keeping you mobile while the car is being repaired. If you agree not to demand this then you may find them more friendly.